1. Field of the Invention
The present invention relates to a method of joining two types of members having different thermal expansion coefficients.
2. Description of the Related Art
In a conventional method of joining two types of members, it is known to join members, for example, different metals or a metal member and a ceramic member, with an Ag wax material.
When pure Ag is used as the Ag wax material, the Ag wax material is placed as a joining layer between the members to be joined. However, when the pure Ag is melted by heating to the melting point of Ag, Ag and other metals are reacted to form alloys, with the result that the joining layer becomes fragile. Then, to join the members at a temperature lower than the melting point of Ag, use of an Ag wax material containing Ag particles in the order of nanometers has been investigated.
As a composition containing the nanometer Ag particles, an Ag-powder mixture is known, which is formed by blending spherical-form Ag powder or flake-form Ag powder and rod-form Ag powder having a lengthwise diameter of not more than 10 μm and a crystallite diameter of not less than 30 nm (see Japanese Patent Application Laid-Open No. 2006-49106). The Ag-powder mixture is blended with a resin component and an organic solvent to prepare Ag paste, which is then used as a means for providing electrical conduction when a circuit such as a circuit of a print wiring substrate is formed.
When the Ag wax material containing nanometer Ag particles is placed between the members to be joined and used as a joining layer, even if the thermal expansion coefficients of the members are smaller than that of Ag, the members can be joined. However, since the joining layer is formed of pure Ag, the thermal expansion coefficient thereof is as large as 19.1 ppm/K. Thus, when the members whose thermal expansion coefficients are smaller than that of Ag are repeatedly subjected to a thermal cycle consisting of heating and cooling steps, the joining layer may not show sufficient durability to the thermal cycle because of the difference in thermal expansion coefficient between the joining layer and the members.